Field of the Invention
The present invention relates to a motor drive device.
Related Art
A motor drive device has been known that drives the motors within machine tools, industrial machines, forge rolling machines, injection molding machines or various robots (for example, refer to Patent Documents 1 and 2). Such a motor drive device includes a forward converter that converts AC power to DC power, a reverse converter that converts the DC power from the forward converter into AC power, and a DC link capacitor provided to a DC link between the forward converter and reverse converter.
However, the DC link capacitor may have a decline in electrostatic capacitance (hereinafter referred to simply as “capacitance”) due to the repeat of charge-discharge. If the capacitance of the DC link capacitor declines, the ripple current flowing to the DC link will increase, and the variation in DC voltage will increase.
Concerning this point, it has been known to detect the capacitance decline of the DC link capacitor using the time-integrated value for the charging current of the DC link capacitor and the voltage value of the DC link capacitor, during the initial charging of the DC link capacitor. Hereinafter, an example of detecting the capacitance decline of the DC link capacitor during the initial charging of the DC link capacitor will be explained.
FIG. 6 is a view showing a circuit configuration for a conventional motor drive device. This motor drive device 1X includes a converter section 10X and an inverter section 20X. The converter section 10X includes a forward converter 11 that converts the three-phase AC power from an AC power source 2 into DC power, and supplies the DC power to the inverter section 20X. The inverter section 20X includes a reverse converter 21 that converts the DC power from the converter section 10X into three-phase AC power, and supplies the three-phase AC power to a motor 3. The inverter section 20X includes a DC link capacitor 22 provided in the DC link between the forward converter 11 and reverse converter 21.
The converter section 10X has a capacitance decline detection function for the DC link capacitor 22. More specifically, the converter section 10X includes current detection parts 13, 14 that detect the AC current inputted to the forward converter 11, a voltage detection part 15 that detects the output voltage of the forward converter 11 (i.e. voltage of DC link capacitor 22 provided to the DC link), a storage part 16 that stores a threshold value for detecting the capacitance decline of the DC link capacitor 22, and a capacitance decline detection part 17. The capacitance decline detection part 17, during the initial charging of the DC link capacitor 22, obtains the capacitance value for the DC link capacitor 22, based on a time-integrated value (electric charge) for the charging current of the DC link capacitor 22 obtained based on the current detected by the current detection parts 13, 14; and the voltage value of the DC link capacitor 22 detected by the voltage detection part 15.
FIG. 7 provides graphs showing the time changes of the charging current and voltage during initial charging of the DC link capacitor in a conventional motor drive device. In FIG. 7, the time t1 is the charging start time, and the time t2 is the time during charging operation. As shown in FIG. 7, initial charging is the charging from when the electric charge of the DC link capacitor 22 is 0 C, and the voltage is 0 V. When defining the voltage value of the DC link capacitor 22 at time t2 as V1, the electric charge of the DC link capacitor 22 at time t2 as Q1, and the time-integrated value for the charging current of the DC link capacitor 22 from time t1 until time t2 as ∫I1dt, the capacitance value C1 of the DC link capacitor 22 is obtained by the following Formula (1).
                              [                      Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                C          ⁢                                          ⁢          1                =                                            Q              ⁢                                                          ⁢              1                                      V              ⁢                                                          ⁢              1                                =                                    ∫                              I                ⁢                                                                  ⁢                1                ⁢                                                                  ⁢                dt                                                    V              ⁢                                                          ⁢              1                                                          (        1        )            
The capacitance decline detection part 17 detects the capacitance decline of the DC link capacitor 22, when the capacitance value obtained using the above Formula (1) is less than the threshold stored in the storage part 16.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2016-178823
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2014-25927